If it is admitted that this liquid is endowed with an absolute fluidity, it is evident that, by giving the globe a brisk rotary movement on a vertical axis, it will turn without carrying the liquid around with it. This may be readily verified by giving it a more or less rapid movement of rotation. Light substances suspended in the water will appear to remain still, despite the ball's rotation. But will this always be the case, whatever the speed of rotation? If the globe were very slowly revolved, would the liquid still be unaffected by the movement of its envelope? In conceding the perfect fluidity of the liquid, its viscosity has been lost sight of. Now, this viscosity, though slight, is not nil; hence, if the rotation be sufficiently slow, the liquid will be carried around with the glass globe, the whole revolving as one piece, or solid ball." This rotation of the liquid under the conditions described has been demonstrated by M. Champagneur in a series of experiments undertaken, by request of M. Delaunay, in the laboratory of the Sorbonne.
Applying this reasoning to the earth, we will assume that it is composed of a liquid mass covered by a solid pellicle. It is first of all evident that, if we set aside the perturbations caused by the equatorial enlargement, the entire mass will turn as one piece on its axis; if any difference whatever could exist between the rate of the envelope's rotation and that of the nucleus, friction will speedily annul it. The perturbing influence of precession and nutation imparts to the solid envelope's proper movement of rotation an extremely slight acceleration. The question is, Does the internal liquid participate in this additional movement, or is the crust only affected by it? "As for me," says M. Delaunay, "there is no room for the slightest doubt. The acceleration due to the causes indicated is so slight that the fluid of the interior must follow the inclosing shell exactly as though the whole were a solid mass. So enormous is the pressure to which the various parts of the liquid mass are subjected that we can form no idea of its effect on the viscosity of the fluid in question. But if that fluid is in the condition of those we are familiar with, what we have described would occur." M. Delaunay concludes by stating that, in his opinion, the phenomena of precession and nutation can furnish no data concerning the greater or less thickness of the earth's crust.
Sir William Thomson again considers the question, and from a new point of view. In theoretically determining the height of the tides, the water only is supposed to yield to the luni-solar attraction—the solid shell of the earth being unaffected by it. Now, it is evident that even an entirely solid sphere will be slightly changed in form by these forces, and that the change will be still greater in a partially liquid sphere. We will first suppose that the entire mass of the globe yields to the attracting forces as readily as if it were liquid. In this case sea and solid land will be raised alike, and, the surface of the sea always being at the same distance from the bottom, no tides will be
